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Advanced image processing
A short guide to tone curve corrections
Image processing is a highly specialized and difficult field that takes years of practice to master.
This basic guide to using tone curves covers a few simple procedures to improve your pictures.
For more about digital-image processing, consult your local book dealer about self-help guides on
this subject.
About the tone curve
The tone curve is a graphic representation of the brightness
and color levels of the image. The bottom axis is the 256 levels
of the original image (input data) from black to white. The verti-
cal axis is the corrected image (output data) with the same
scale from top to bottom.
The bottom left portion of the graph represents the dark colors
and shadow areas of the image. The middle section represents
the mid-tones: skin, grass, blue sky. The top right section is the
highlights: clouds, lights. Changing the tone curve can affect
the brightness, contrast, and color of the image.
Input
Output
Highlights
Mid-
tones
Shadows
Bring out detail in the shadows
This is a simple technique to make a subject hidden in
the shadows brighter. Unlike the brightness level control
(p. 26), this method of correction will not loose details in
the highlight areas of the image.
With the RGB channel selected, place the smooth-curve
cursor on the center of the curve. Click and drag the
curve up. Look at the displayed image to judge the result.
The adjustment can be very small and still have a signifi-
cant impact on the image. Moving the tone curve down
will make the image darker.
